This invention relates to nip force controlling devices, and more particularly to a roll fusing apparatus in electrostatographic reproduction machines including a life-extending fusing nip force controlling device.
In industry, substrate handling machines in which paper, film, or other thin substrate material are conveyed, typically use a combination of at least a pair of rolls or rollers that are pressed against each other to form a nip by which, and through which the thin substrate material is conveyed.
In electrostatographic reproduction machines which produce toner copies of images, a pair of fusing rollers, (one of which is heated and the other a pressure roller) are used to form a fusing or fusing nip. Loose powder toner images formed on a copy substrate, such as a copy sheet of paper, are fed through the fusing nip in order to heat, fuse and permanently fix the toner image to the copy substrate. The fusing nip force is very important. If the nip forces are high and thus the overall pressure (force over area) is too light, there will not be enough heat transfer to adequately bond the toner to the paper, this will cause the print to deteriorate. Also, if there is an uneven pressure distribution along the roll, the paper will not track straight as it passes through the rolls. For best fusing results, it is desirable to have close control over the temperature and pressure applied to, as well as the dwell time within the nip of, each unit area of toner powder images being fused and fixed. To achieve such control, the nip force of the fusing nip must be set accurately during machine manufacture, as well as later on, on a regular basis, in the field due to wear, and to the effect of machine jams.
In general, as two nip-forming cylindrical rolls or rollers are pressed against each other to form a nip, there exists a two-dimensional plane or area of contact between the rollers. In electrostatographic reproduction machines, the geometry of this contact area directly impacts the quality of the fused toner images reproduced. In particular, as each copy sheet travels through the fusing nip, its path of travel may vary if the contact geometry is not symmetric along the center of the rollers. A non-symmetric contact geometric area ordinarily results in either sheet skew, or copy damage. In addition, if the nip geometry is out of tolerance, the powder toner may not be fused properly onto the copy sheet.
Conventionally, fusing nip forces, gaps and pressures in electrostatographic reproduction machines are set in the factory or in the field by trained operators using static measuring devices or tools. For example, fusing nip widths or footprints may be measured using a powder-on-roll indicator, or some visual scale for gauging the nip width. Adjustments require that adjustment screws be turned in a trial and error manner during manufacturing or in the field turn in attempts to obtain acceptable nip width, force or pressure. Accordingly, such trial and error methods usually require several iterations of the measure and adjust cycles, and typically depend on subjective operator feel and judgment.
Because the fusing nip force can significantly impact copy quality, there exists a need for a device or assembly to control it. Some known devices for making measurements within roll nips, in order to achieve and maintain close control, are disclosed for example in the following references. U.S. Pat. No. 3,760,637 issued Sep. 25, 1973, to Budinger et al., for example, discloses a tool for measuring the pressure exerted at the nip between two rolls. The tool includes a thin-walled, non-rigid tube, a fluid conduit, means for pressurizing fluid passed through the conduit, and means for measuring the fluid pressure when it is balanced by the nip pressure.
U.S. Pat. No. 3,906,800 issued Sep. 23, 1975, to Thettu, discloses a reusable nip measuring device and method. The device consists of two polyimide sheets each having a thickness of 3 to 8 microns, and one of which is coated with silicone rubber material. When placed in the heat and pressure nip of a fuser for a determined period, the silicone rubber sheet forms an impression on the uncoated sheet corresponding to the contact arc of the nip.
U.S. Pat. No. 3,926,058 issued Dec. 16, 1975, to Thettu, discloses a device for measuring the contact arc and pressure characteristics of a roll fuser nip. The device consists of silicone rubber layer into which a toner powder pattern is formed and fused, and of a sheet of paper placed of the toner powder pattern. When the device as arranged is placed in the heat and pressure nip of a fuser for a determined period, a portion of the powder pattern corresponding to the contact arc is offset onto the sheet of paper.
U.S. Pat. No. 4,397,097 issued Aug. 9, 1983, to Damrau et al. discloses a gauge for measuring the size of a roll nip. The gauge includes a cylindrical carrier enclosing a pivotal platform. It also includes a U-shaped rod or probe and a transducer mounted on the platform. In use, the medial portion of the U-shaped probe is moved into the nip until the carrier contacts the rolls, so that the transducer can give a readout in accordance with the radii of the rolls at the point of contact by the probe.
U.S. Pat. No. 4,744,253 issued May 17, 1988, to Hermkens discloses a system for determining the pressure in the nip between two rollers. The system includes a pressure sensor and a device for transmitting an ultrasonic wave in the sensor, and receiving a reflected pulse thereof. According to the system, a time difference between a transmission pulse and its reflected pulse is related to the pressure exerted on an object in the nip.
Xerox Disclosure Journal, Vol. 15, No. 4, July/August 1990 discloses a fuser nip length sensor consisting of a thin profile linear potentiometer. The potentiometer includes a voltage divider consisting of a strip of resistive material, a series of taps therealong, force sensitive switches and a common terminal. When placed longitudinally within the nip, only a portion of the potentiometer corresponding to the length of the nip will be compressed.
Xerox Disclosure Journal, Vol. 15, No. 6 November/December 1990 discloses a tool for measuring fuser nip pressure. The tool includes short force sensing resistors placed over metallic conductors constructed in an interdigitated pattern and mounted between two polymer sheets forming a network. Two metallic strips running the entire length of the tool connect the network to a measurement device. When the tool is inserted in a nip, pulses from the force sensing resistors are timed, and the time data is used along with fuser speed to calculated nip force.
Xerox Disclosure Journal, Vol. 18, No. 4 July/August 1993 discloses a fuser nip sensor for determining nip pressure and nip length. The tool also includes short force sensing resistors in addition to two sensor heads placed in proximity to the nip for determining nip length.